In this study, we assessed the influence of superoxide dismutase, which was varied by inducing its synthesis or by adding it exogenously, on killing of black-pigmented oral anaerobic rods by neutrophils. Porphyromonas gingivalis 381 which had a high SOD activity was not highly sensitive to the killing effect of neutrophils. However, Prevotella denticola ATCC 33185 which showed the lowest activity of SOD and was also the highest oxygen sensitivity among the black-pigmented oral anaerobic rods tested was immediately killed after phagocytosis.SOD_s were purified from extracts of either anaerobically maintained or aerated P. gingivalis 381 (anaers-SOD and aero-SOD, respectively). Each-purified enzyme showed same molecular weight and isoelectric points. Furthermore, the two enzymes had completely identical amino acid, -sequences. However, metal contents and spectral analysis of both enzymes showed that anaers-SOD had the characteristic of FE-SOD and that aero-SOD had that of MnSOD. The effect of aero-SOD or anaers-SOD added exogenously on resistance of P. gingivalis to killing by neutrophils was examined. Both SOD_s inhibited bacterial killing, but there was no difference in the inhibitory effects between these two enzymes.Induction of the synthesis of SOD in P. gingivalis 381 was performed by an aeration or by the addition of potassium nitrate to the growth medium. When the specific activities of SOD in P. gingivalis 381 increased 2.3-fold and 2.0-fold by growth in the presence of nitrate and by aeration, respectively, the resistances of the bacterium to killing by the neutrophils were significantly greater than that of control. These results suggest that SOD may form part of a defence mechanism that helps protect P. gingivalis from killing by neutrophils.以上のことからP.gingibalisのもつDODが白血球などの貪食細胞から放出されるO_2^ーから菌体を保護する機能を果していることが明らかにされた。